1. Field of the Invention
The present invention is related to medical equipment and, more specifically without limitation, to respiratory apparatus.
2. Definitions
As used herein, the following definitions and abbreviations have been applied:                PFT is an abbreviation for Pulmonary Function Test as hereinafter described;        TV is an abbreviation for Tidal Volume, which is defined as the volume of air that a subject breathes in and out while resting;        FVC is an abbreviation for Forced Vital Capacity, which is defined as the volume of air that a subject expels from maximum inhalation to maximum exhalation;        ERV is an abbreviation for Expiatory Reserve Volume, which is defined as the volume of air a subject expels from normal exhalation to maximum exhalation;        IRV is an abbreviation for Inspiratory Reserve Volume, which is defined as the volume of air a subject inhales from normal exhalation to maximum inhalation;        FRC is an abbreviation for Functional Residual Capacity, which is the sum of ERV and RV;        RV is an abbreviation for Residual Volume, which is the difference between FRC and ERV;        TLC is an abbreviation for Total Lung Capacity, which is the sum of SVC and RV;        DLCO is an abbreviation for Diffusion Lung Capacity of Carbon Monoxide, which is the quantity of carbon monoxide that is diffused from the lungs to the blood stream.        
3. Discussion of the Related Art
PFT equipment typically includes three gas cylinders: (I) a cylinder that contains 100% pure oxygen, (ii) a cylinder that contains 3% carbon monoxide, and (iii) a cylinder that contains 10% helium. The PFT equipment also includes three independent analyzers: (I) either a nitrogen analyzer or an oxygen analyzer, (ii) a carbon monoxide analyzer, and (iii) a helium analyzer.
The primary purpose of PFT equipment is to measure the function and the activity of a human subject's lungs. The resulting test data is then compared to expected data correlated to the height, weight, age, and sex of the human subject being evaluated. A physician utilizes the PFT results to make a diagnosis and prescribe medication and treatment. In other words, it is absolutely imperative that collected PFT data be correct and accurate.
A PFT is used to assess potential pulmonary problems prior to surgery. A complete PFT includes the following procedures: (1) determination of the total lung capacity (TLC) of a subject's lungs; (2) determination of the tidal volume (TV) of the subject; (3) determination of the slow vital capacity (SVC) of the subject; and (4) determination of the expiatory reserve volume (ERV) of the subject. If the pre-operative respiratory status at least meets minimum acceptable PFT requirements, the subject is considered to be at a lower risk for post-operative complications. At times, a subject's surgery may be delayed or cancelled if the PFT results do not meet acceptable minimums.
In addition to the foregoing, PFT results are also used to assess impending respiratory failure in myasthenia gravis and Guillian-Barré patients. Such subjects may have neuromuscular disorders that can cause extreme weakness in, or paralysis of, the respiratory system. The measurements of vital capacity and negative inspiratory force are generally utilized as an integral part of the diagnosis for determination of respiratory failure. These tests can be performed at time intervals ranging from, for example, every hour to every eight hours to determine whether the patient may be subject to impending respiratory failure, which is generally demonstrated by a decreasing FVC value.
PFT results can also be used to assess a subject's response to bronchodilator therapy. Pre- and post-bronchodilator testing of the FVC can generally provide a reliable indication of the effectiveness of a bronchodilator. A determination can then be made as to the type of bronchodilator to be used, the preferred frequency of treatments, and the dosage amount that is appropriate for each particular patient.
PFT tests have been proven to be an effective means of evaluating the pulmonary status of a subject. PFT tests have been utilized for decades and continue to constitute a preferred state-of-the-art diagnostic procedure. Equipment and procedures for PFT tests are continuously being updated and improved to meet the constantly changing needs of pulmonary subjects.
Most PFT procedures are performed by a respiratory therapist (RT) under the direction of a physician. An RT is a professional who is thoroughly trained in PFT testing procedures and is skilled in calibrating PFT equipment, performing PFT tests, and interpreting PFT test results. Any abnormal results observed in a PFT are immediately communicated to the tending physician in order to provide the best possible care for the subject.
Presently, PFT equipment is calibrated by using a three-liter syringe for measuring gas volume. An RT normally conducts such calibrations by comparing two tests, a diffusion test and a quality control test, neither of which is overly reliable.
Equipment calibration involves the cleaning, maintenance, testing, and adjusting of various parameters. Equipment can lose calibration either if used frequently between calibrations, or if used too infrequently. If not frequently recalibrated, particulate matter may build up inside the equipment thereby contributing to operational inaccuracy of the equipment. It is therefore essential to calibrate PFT equipment regularly in order to ensure validity of test results. Each manufacturer of PFT equipment establishes recommended initial calibrations and provides specifications for the recommended frequency of calibration and the quantitative results to be expected during calibration of its PFT equipment. Hospital policies also establish calibration standards for PFT equipment used in its facilities.
Prior art equipment for PFT calibration, which can be described as follows, includes a cylindrically shaped syringe 10 having an interior cavity 12 with a capacity of three liters. The purpose of the syringe 10 is to mechanically duplicate the activity of the lungs of a human subject in order to calibrate TV, SVC, RV, ERV, FRC, DLCO, and TLC determinations of PFT equipment. A distal end 14 of the syringe 10 is removably connected via conduit 16 in flow communication with existing PFT equipment 18. The syringe 10 includes a plunger 20 having a piston-type head 22 mounted on a shaft 24. The shaft 24 slidably engages a centrally located orifice 26 through a proximal end 28 of the syringe 10. A handle 30 on the end of the shaft 24, opposite the head 22, provides means for manually displacing the head 22 back and forth within the interior cavity 12 of the syringe 10 as indicated by the arrow designated by numeral 32 in FIG. 2.
The plunger shaft 24 has indicia 34 and demarcations 36 thereon, as depicted in FIG. 3, which become visible as the plunger shaft 24 is pulled slidingly outwardly from the syringe 10. As the plunger 20 is positioned such that a selected one of the demarcations 36 is aligned with an outer surface 38 of the proximal end 28 of the syringe 10, an expellable volume of gas, as indicated by the indicia 34 associated with that particular demarcation 36, is captured between the head 22 and the distal end 14 of the internal cavity 12 of the syringe 10. If the plunger 20 is then displaced such that a different demarcation 36 is aligned with the outer surface 38 of the proximal end 28 of the syringe 10, the net volume change of the internal capacity 12 captured between the head 22 of the distal end 14 of the syringe 10 by displacing the plunger 20 from one to the other of those two selected demarcations 35 is the difference between the expellable volumes corresponding thereto. Generally, the volumes defined by the demarcations 36 are calibrated in one-half liter increments ranging from one-half liter to three liters.
Calibration of PFT equipment 18 includes a procedure commonly referred to as a DLCO test. A collapsible, variable capacity bag 40, commonly referred to as an FRC bag 40, having a capacity of one and one-half liters, is connected to a T-adapter 42. T-adapter 42 includes a restrictor 46 that prevents gas from flowing through T-adapter 42 until the gas in the bag 40 has been depleted. A gas source 48, such as a tank of 3% carbon monoxide, and a carbon monoxide analyzer 50 are connected in flow communication with the PFT equipment 18 as schematically indicated in FIG. 2. The PFT equipment 18 provided by some manufacturers utilizes an oxygen analyzer as analyzer 50 instead of a nitrogen analyzer which is only used in nitrogen wash-out test which measures FRC; however, the actual calculations are essentially identical.
To begin a nitrogen washout test, the FRC bag 40 is filled with atmospheric air (seventy-nine percent nitrogen, twenty-one percent oxygen); the gas source is 100% oxygen. The plunger 20 is then displaced to cause one-half liter increments of pure oxygen to flow from gas source 48, into the FRC bag 40. Outflow of gases from the FRC bag 40 is directed to analyzer 58 whereupon the percentage of nitrogen is determined by analyzer 50, and that percentage is mathematically converted to volume of nitrogen with each stroke of the plunger 20. The nitrogen washout test is complete when the analyzer 50 determines that the outflow into the analyzer 50 contains zero-percent nitrogen indicating that the atmospheric air in the FRC bag 40 has been replaced by pure oxygen. The sum of the successive volumes of nitrogen corresponding to the strokes of the plunger 20 is the total volume of the FRC bag 40. With this data from the nitrogen washout test, two values have been determined: VT and FRC. This is an indirect measurement.
Calibration of the PFT equipment 18 also includes a procedure commonly referred to as a helium dilution test. The only difference between the helium dilution test and the nitrogen washout test is that gas source 48 is a tank of ten-percent helium instead of a tank of pure oxygen, and analyzer 50 is a helium analyzer instead of a nitrogen or oxygen analyzer. The test is complete when the analyzer 50 determines that the outflow into the analyzer 50 contains the same % helium as the inflow percent of helium, indicating that the FRC bag 40 is free of atmospheric air and, instead, has the same percent of helium, sometimes referred to as equibilation. With this data from the helium dilution test, total volumes for FRC and TV are determined. The PFT equipment 18 provided by some manufacturers utilize one or the other, or both, tests for indirect measurement; however, both give essentially the same results.
Calibration of PFT equipment 18 also includes a procedure commonly referred to as an SVC test. This test is a single stroke test wherein the plunger 20, by means of the indicia 34 and demarcations 36, is displaced to expel two and one-half liters of gas from the syringe 10 into the PFT equipment 18.
Calibration of PFT equipment also includes a procedure commonly referred to as a DLCO test. The FRC bag 40 is not utilized for this test but, instead, is replaced by a fixed capacity bag 52, commonly referred to as a DLCO bag 52. Dilution occurs when gases are mixed; diffusion occurs when oxygen moves from the lungs to the blood system. Carbon monoxide is used because it has a greater propensity for blood. Prior to performing the diffusion test, a carbon monoxide (CO) analyzer is utilized as analyzer 50, and is calibrated to the percentage of gas in gas source 48, which for this test comprises a tank containing 3% carbon monoxide. Atmospheric air in the DLCO bag is allowed to mix with the three-percent carbon monoxide. After a twelve-second delay inside the three-liter syringe 10, the mixture is expelled into the PFT equipment 18 to be analyzed by carbon monoxide analyzer 50. Since the percent of carbon monoxide at the beginning of the test is known and since the carbon monoxide analyzer 50 determines the percent of carbon monoxide at the completion of the test, the difference between these two percentages provides the desired quantity.
It should be obvious that the accuracy and reproducibility of calibration of PFT equipment is highly user-dependent as each user may align the demarcations 36 differently each time the syringe is used. Further, each different user using the same syringe may align the demarcations 36 differently from that of other users.
What is needed is a system for calibrating PFT equipment that is more efficient, reduces the probability of error, and provides consistent results when used by multiple users.